NCERT Solutions for Class 9 Science Unit 8

Motion Class 9

Unit 8 Motion Exercise Solutions

Motion:- If an object changes its position with respect to a reference point with elapse of time, the object is said to be in motion. Rest:- When an object does not change its position with respect to a reference point with elapse of time, the object is said to be in rest. Example When a vehicle changes its position with respect to an electric pole (a reference point) with elapse of time, then vehicle is called in motion. And if the same vehicle does not change its position with respect to that electric pole, the vehicle is called in the state of rest. Thus to observe the motion of any object; two things are necessary a reference point and time.

(b)Its magnitude is greater than the distance
travelled by the object.

Answer :

(a)Not true

Displacement can become zero when the initial
and final position of the object is the same.

(b)Not true

Displacement is the shortestmeasurable
distance between the initial and final positions of an object. It
cannot be greater than the magnitude of the distance travelled by
an object. However, sometimes, it may be equal to the distance
travelled by the object.

During an experiment, a signal from a spaceship reached the
ground station in five minutes. What was the distance of the
spaceship from the ground station? The signal travels at the
speed of light, that is, 3
ÃƒÆ’Ã¢â‚¬”
108m
s - '1.

A stone is thrown in a vertically upward direction with a
velocity of5 m
s - '1.
If the acceleration of the stone during its motion is 10 m
s - '2in
the downward direction, what will be the height attained by the
stone and how much time will it take to reach there?

Joseph jogs from one end A to the other end B of a straight 300 m
road in 2 minutes 30 seconds and then turns around and jogs 100 m
back to point C in another 1 minute. What are
Joseph's
average speeds and velocities in jogging (a) from A to B and (b)
from A to C?

Abdul, while driving to school, computes the average speed for
his trip to be 20 km
h-1. On
his return trip along the same route, there is less traffic and
the average speed is 40 km
h-1.
What is the average speed for
Abdul's trip?

A driver of a car travelling at 52 km
hÃƒÂ¢Ã‹â€ '1
applies the brakes and accelerates uniformly in the opposite
direction. The car stops in 5 s. Another driver going at 3 km
hÃƒÂ¢Ã‹â€ '1
in another car applies his brakes slowly and stops in 10 s. On
the same graph paper, plot the speed versus time graphs for the
two cars. Which of the two cars travelled farther after the
brakes were applied?